A catalyst for purification of automobile exhaust gas has been conventionally developed. With tightening of regulations on automobile exhaust gas, the exhaust gas purification ability needs to be enhanced. In particular, a catalyst capable of achieving high exhaust gas purification performance under various engine use conditions is required.
For example, maintaining of exhaust gas purification performance over an extended period of time is required of an automobile exhaust gas purification device. This shows that an increase in lifespan of a catalyst for purification of exhaust gas is increasingly required. For the enhancement of the long-term durability of a catalyst, suppression of sintering of noble metal particles supported on the catalyst and suppression of sulfur poisoning are investigated. On the other hand, it is known that poisoning due to phosphorus contained in exhaust gas (hereinafter, referred to as “phosphorus poisoning”) largely affects deterioration in catalytic performance (Non-Patent Literature 1). In order to satisfy further tightening regulations, suppression of phosphorus poisoning or enhancement of catalytic performance after phosphorus poisoning is important. In recent years, not only suppression of phosphorus poisoning but also suppression of deterioration in catalytic performance due to phosphorus poisoning even under exposure to exhaust gas of high temperatures of 950° C. or higher are required. Therefore, both the deterioration in catalytic performance due to sintering and the deterioration in catalytic performance due to phosphorus poisoning are required. Accordingly, the increase in lifespan of catalytic performance is strongly required as compared to the conventional catalyst.
As a substance causing phosphorus poisoning, a phosphorus compound derived from a lubricant additive, such as zinc dialkyl dithiophosphate, which is contained in exhaust gas, is known. When the exhaust gas flows through a purification device, the phosphorus compound contained in the exhaust gas is deposited on a catalyst layer and penetrates into the catalyst layer, resulting in phosphorus poisoning. The deterioration in catalytic performance due to such phosphorus poisoning is known (Non-Patent Literature 1).
It is known that the deterioration in catalytic performance due to phosphorus poisoning is caused by the following phenomenon. Due to the phosphorus compound that is deposited on and penetrates into the catalyst layer, diffusion of exhaust gas in the catalyst layer is inhibited. Further, cerium oxide as an oxygen occlusion and release material (also referred to as oxygen storage material) that is often used for a three-way catalyst reacts with the phosphorus compound to form cerium phosphate. Once the cerium phosphate is formed, occlusion and release of oxygen are stopped. Therefore, when exhaust gas atmosphere is changed to a lean or rich state, the effect of relaxing the lean or rich state due to occlusion and release of oxygen does not occur. Due to the occurrence of phenomena such as diffusion inhibition and inhibition of occlusion and release of oxygen, the exhaust gas purification rate is decreased.
Patent Literature 1 discloses that a composite oxide of ceria with zirconia is used for a catalyst using palladium in order to suppress phosphorus poisoning.
Patent Literature 1 discloses a catalyst using palladium in which the performance is likely to be deteriorated by phosphorus poisoning, and focuses on the easy formation of cerium phosphate due to ceria. Patent Literature 1 discloses that deterioration in catalytic performance due to phosphorus poisoning is suppressed compared to a conventional technique by replacing ceria with the composite oxide of ceria with zirconia. In Patent Literature 1, the deterioration in catalytic performance due to the phosphorus poisoning of a catalyst using rhodium that is the most active as a three-way catalyst is not investigated.
Patent Literature 2 discloses that deterioration in performance due to phosphorus poisoning is suppressed by providing a zone where a catalytic material is not applied, as a phosphorus capture zone on an upstream end of a catalyst.
A large amount of phosphorus compound is attached to an upstream side of a catalyst in the exhaust gas flow direction. As a countermeasure for the attachment, the catalyst described in Patent Literature 2 has a phosphorus capture zone that does not include a noble metal on the upstream end. However, Patent Literature 2 does not disclose catalytic performance after phosphorus poisoning. The effect thereof is not clear. When the length of the catalyst between the upstream end and a downstream end is relatively short, the phosphorus compound may be attached to the vicinity of the downstream end of the catalyst and deteriorates performance. Therefore, the long-term durability is not sufficient.
Patent Literature 3 discloses a catalyst in which a single layer of only a palladium-supporting layer is provided on an upstream end of a lower catalyst layer, an upper catalyst layer is laminated on a downstream side, and rhodium is supported.
Patent Literature 3 discloses that by providing a single palladium layer region on an upstream side of the catalyst, exhaust gas is likely to be diffused toward the lower catalyst layer. However, in Patent Literature 3, phosphorus poisoning is not investigated. The performance of the catalyst described in Patent Literature 3 under a condition where a phosphorus compound is contained in the exhaust gas is not clear.
Patent Literature 4 discloses a catalyst in which a lower catalyst layer is provided on a substrate over the whole between an upstream end and a downstream end, an upper catalyst layer is laminated on the lower catalyst layer on the upstream side and the downstream side, and a surface of the lower catalyst layer is exposed without laminating the upper catalyst layer at a middle portion between the upstream side and the downstream side. However, Patent Literature 4 does not disclose the overall length of the catalyst and a specific value for the middle layer. Further, Patent Literature 4 does not disclose that an object is to decrease the thermal capacity of the catalyst, decrease the resistance to the gas stream, and decrease a catalyst component, and catalyst poisoning due to phosphorus in exhaust gas can be improved by the relationship between the overall length of the catalyst and the middle portion.